In electrophotographic copying machines based on Carlson's method, charging the surface of a photoreceptor is followed by exposure for imagewise charge removal to form an electrostatic latent image, which is developed with a toner, and the resulting visible image is transferred and fixed onto a transferee such as paper.
At the same time, the photoreceptor is treated to remove the adhering toner, eliminate the carrier and clean the surface to ensure long-term repeated use.
Therefore, the electrophotographic photoreceptor is required to be good in physical properties such as printability, wear resistance and moisture resistance in repeated use and resistance to ozone generated upon corona discharge and to ultraviolet rays generated upon exposure (environmental resistance), as well as in chargeability and electrophotographic properties such as high sensitivity and low dark decay.
Conventional electrophotographic photoreceptors in common use are inorganic photoreceptors having a light-sensitive layer based mainly on an inorganic photoelectrically conductive material such as selenium, zinc oxide or cadmium sulfide.
In recent years, there has been a trend toward development of organic electrophotographic photoreceptors of high sensitivity and high durability wherein the carrier generation and carrier transport functions are allotted to different substances in a light-sensitive layer, which substances are selected over a wide range to exhibit the respective functions according to the desired characteristics.
Such organic photoreceptors of the separate function type have traditionally been used mainly for negative charging; a thin carrier generation layer is formed on the support, on which a relatively thick carrier transport layer is formed.
As a binder for such photoreceptors, polycarbonate of the bisphenol A type represented by the following structural formula is well known to offer good properties in terms of chargeability, sensitivity, residual potential and durability in repeated use. ##STR2##
Polycarbonates of the above structural unit are hereinafter referred to as of the BPA type. This type of polycarbonate has a structure wherein two methyl groups are symmetrically bound to the central carbon atom of bisphenol A. Investigations have revealed, however, that this type of polycarbonate has the following drawbacks:
(1) Mechanical strength, especially flaw resistance and wear resistance are not sufficient to allow satisfactory improvement in organic photoreceptor durability.
(2) Poor compatibility with carrier transport material (CTM) easily results in CTM crystal separation, which can lead to cracking and other troubles in the coating film.
(3) Using a hot coating solution for dip coating etc. tends to cause coating solution gelation.
(4) Gel projection on the light-sensitive layer surface causes toner filming upon cleaning, which is likely to result in an imaging failure.
(5) Severe abrasion by cleaning blade etc. hampers the improvement of organic photoreceptor durability.
Herein after referred to as Japanese Patent Publication Open to Public Inspection No. 172045/1985 propose non-crystalline polycarbonates such as those having a bulky substituent and those of the Z type wherein a ring has been formed at the central carbon atom. The use of the polycarbonate having a bulky substituent or the polycarbonate of the Z type eliminated the above-described drawback in the polycarbonate of the BPA type. However, due to photoreceptor fatigue deterioration by ozone generated upon corona discharge and other active substances generated upon imagewise exposure, durability in long-term image formation was insufficient. To overcome this problem, Japanese Patent O.P.I. Publication Nos. 118137/1989, propose the addition of an antioxidant comprising a compound having a hindered phenolic structural unit and/or a hindered amine structural unit to the polycarbonate having a bulky substituent, or a polycarbonate of the A or Z type, or a polycarbonate resulting from copolymerization of these polycarbonate structural units.
Japanese Patent O.P.I. Publication No. 20768/1991 proposes a method for improving the blade cleaning property by using as a binder a polycarbonate resin of the bisphenol Z type on the surface of the light-sensitive layer. The same publication describes improvements in light-sensitive material's resistance to wear by the cleaning blade, ozonic deterioration during charging and other film properties of the light-sensitive layer.
However, with the popularization of copying machines, there is demand for improved image quality, high copying speed and high durability.
Particularly the OPC photoreceptor is more sensitive to light-sensitive layer wear in multiple imaging, so that the electrophotographic performance is liable to deteriorate, in comparison with other types of photoreceptors such as those of selenium; there is demand for the development of a binder resin having still better wear resistance. In addition, in manufacturing a copying machine, cleaning elements, such as the cleaning blade, are usually produced pressed against the electrophotographic photoreceptor, and its storage, transport, etc. often require a long period of several months or sometimes over 1 year, until it is used by a user. This poses a problem of image failures such as black streaks and white streaks due to cracking upon imaging as a result of the action of unreacted components, such as polyol, of the cleaning blade etc. on the light-sensitive layer.
Meantime, since high image quality and good copying workability depend also on surface smoothness and evenness in the photoreceptor of uniform thickness, the coating composition for constituting the coated structural layer of the photoreceptor and film failures such as orange peel, pinholes, coating streaks and solvent cracks occurring upon coating or drying are of major concern from the viewpoint of copying performance and production efficiency.
Surfactants are also useful in improving surface quality or lubrication and also effective in improving suspensoid dispersibility and dispersion stability in the case of coating suspensions and valuable in promoting dissolution and improving coatability and other properties related to productivity in the case of coating solutions. However, a mistakenly chosen surfactant often causes poor layer-to-layer adhesion, deterioration-related failures or troubles due to a lack of moisture resistance.
To overcome these drawbacks, there have been various proposals, including, use of a copolymer of monomers resulting from substitution of both phenylene rings with a phenyl group or a cyclohexyl group (Japanese Patent O.P.I. Publication Nos. 269942/1989) and use of distyryl, as a carrier transport material, in combination with a polycarbonate of the bisphenol Z type (Japanese Patent O.P.I. Publication No. 32265/1989). However, there remain some problems, such as insufficient surface strength and surface smoothness, a lack of wear and crack resistance, image quality deterioration in repeated use, and sensitivity deterioration due to wear-related film thinning.